Efforts to improve air quality have lead to many government issued emission controls and regulations. Over the past 30 years, the allowable emissions from spark ignited gasoline engines have been substantially reduced. In tension with the efforts to reduce emissions is a general desire for increased fuel efficiency. This is particularly a problem with diesel engines, which while being very efficient often produce very high emissions of both NOx and particulate matter (“PM”). Indeed, in order to be compliant with gasoline spark ignited engine regulations, the emissions from a modern diesel engine must be reduced by a factor of around 10 to 50, depending on specific engine type.
Lean-burn engines include both spark-ignition (“SI”) and compression-ignition (“CI”) engines. In comparison to conventional SI engines, lean-burn SI engines offer 20-25% greater fuel economy, while CI engines offer 50-100% greater fuel economy. CI engines are widely used throughout the United States in heavy-duty vehicles and their use in light-duty vehicles is expected to grow. CI engines are also widely used throughout much of the world in passenger vehicles, light and heavy-duty trucks, and electric power generators.
Currently, automotive emission control is based largely on three-way catalyst technology, which reduces the emissions of carbon monoxide (“CO”), unburned hydrocarbons (“UHCs”) and NOx. This technology can be highly effective for ordinary gasoline engines operating at stoichiometric, or near stoichiometric air/fuel ratios. However, three-way catalyst technology is generally not suitable to lean burn engines, which generate a very lean overall exhaust mixture containing a large excess of oxygen. This is because the excess oxygen in the exhaust impedes the reduction of NOx, a major limitation of both lean-burn engines and TWC-based emission control technology. For example, in the case of lean-burn diesel engines, the emission control system must remove NOx and PM from an exhaust stream containing about 6-15% excess oxygen.
Many technologies have therefore been explored to address the problem of NOx removal from lean-burn engine exhaust. One such technology utilizes NOx storage-reduction (“NSR” or NSR-type) systems. This technology has been described throughout the scientific literature and is generally well known in the art. For example, description of this technology may be found in S. Matsumoto, CATTECH, Vol. 4, No. 2, pp. 102-109, 2000, and the references cited therein, all of which are hereby incorporated by reference in their entirety.
As described therein, a typical NSR catalyst has an adsorbent-catalyst system, providing the dual functions of reversible NOx storage or trapping, and NOx reduction. One component of the NSR catalyst reacts with NOx in the gas stream to capture it under oxidizing conditions or conditions where the exhaust stream contains excess O2. This component is selected so that when the exhaust stream is made reducing, that is, containing excess reducing species, the NOx is released. The NSR catalyst also contains a NOx reduction catalyst that reacts NOx with a reducing agent under reducing conditions to form non-polluting N2. When the exhaust stream is made reducing, the NOx is released and this NOx reacts with the reducing species on the NOx reduction catalyst to form N2. One example of an adsorbent-catalyst system is the Pt/Rh/JBa/TiO2/ZrO2/γ-Al2O3 system, which has been used commercially in vehicles in Japan.
Advantages of the NSR catalyst include its compatibility and effectiveness with fuel-efficient lean-burn IC engines; its commercial acceptance; its unneeded use of ammonia or urea as reducing agents; and its ability to obtain high NOx conversions when operated at ideal conditions. For example, NOx conversions of 90 to 100% have been achieved in tests of diesel engines under ideal conditions using diesel fuel as a reducing agent.
However, NSR technology has some serious disadvantages and limitations as well. During regeneration of the NSR catalyst, the environment of the NSR catalyst must be made rich to convert the trapped NOx to N2 and to regenerate the catalyst. If the reducing environment in the exhaust were to be obtained by modifying the engine operation from lean to rich, then the engine cycle would be operating in a region where it was not designed to operate. For example a diesel engine, which usually operates without a throttle on the air intake, would now require a throttle to drive the air/fuel ratio into the rich regime. In addition, this would have to be done quickly and quite frequently, from about every 2 to 20 minutes depending on a variety of operating conditions.
Fuel may be injected into the exhaust stream and combusted on the NSR catalyst or on an upstream oxidation catalyst in order to both consume the oxygen and to produce the reducing environment. At high exhaust temperatures, this has been shown to give reasonable regeneration cycles and NOx conversion efficiency. At low load and low exhaust temperatures, however, this procedure does not work well since the catalyst is not sufficiently reactive with diesel fuel. In addition, the high temperatures produced could drive the NSR catalyst to an undesirably high temperature.
Another disadvantage of NSR technology is that NSR adsorbents are typically very sensitive to sulfur. The NOx adsorbent material can react with sulfur oxides contained in the fuel to form sulfates, as described for example in S. Matsumoto, CATTECH, Vol. 4, No. 2, pp. 102-109, 2000; K. Yamazaki et al., Applied Catalysis B: Environmental, Vol. 30, Nos. 3 and 4, pp. 459-468, 2001 and the references cited therein, all of which are hereby incorporated by reference in their entirety. These sulfates are not readily decomposed and slowly convert the NOx adsorbent to an inactive sulfate, reducing its trapping efficiency. Even with low sulfur fuel (e.g., in the range of 15 ppm) the NSR catalyst only lasts for about 500 to 1000 vehicle miles before its NOx trapping efficiency becomes significantly reduced. To produce a NSR catalyst that would last the 150,000 to 400,000 miles required by current and foreseeable emissions regulations, the NSR unit would have to be designed much too large to be conveniently utilized within the industry. While it has been found that the NSR catalyst unit can be desulfated by treatment in a reducing atmosphere at temperatures of 500-650° C., it is very difficult to operate an engine in a manner so as to produce this environment within the NSR trap unit itself.
Use of diesel fuel as a reductant by direct injection into the exhaust is not very effective at exhaust stream temperatures from 150-250° C., which covers a significant portion of the operating cycle of a diesel engine, including idle and low load. While high NOx conversions may be possible using a diesel reductant at high inlet temperatures (e.g., ranging from 250-300° C. or above), these temperatures are often unobtainable over a sufficiently wide spectrum of operating conditions to make this a useful approach. Other reducing agents, such as hydrogen, carbon monoxide, alcohols and some low molecular weight hydrocarbons (e.g., propylene), are more reactive at low temperatures and may provide better reducing capacities within a wider range of operating conditions. However, use of these materials would require the vehicle have an additional fuel source, posing significant infrastructure and design implications as well as increased costs. These disadvantages and limitations have largely prevented widespread commercial use and acceptance of NSR technology.